1. Field of the Invention
This invention relates to scanning probe microscopes, particularly the scanning tunneling microscope (STM) and the atomic force microscope (AFM). Much extra information not obtainable with conventional STMs and AFMs is to be gained from studying samples that are either heated or cooled. However, the extreme dimensional stability required for successful operation of a scanning probe microscope limits the degree to which heating or cooling can be carried out, because thermal expansion or contraction of the microscope during the STM or AFM scanning process cannot be tolerated, even at an atomic level. This invention relates to a simple method for heating or cooling samples in a scanning probe microscope while retaining stable STM and/or AFM operation, even at the atomic level of resolution.
2. The Prior Art
The first successful operation of a variable temperature microscope was carried out in ultrahigh vacuum. An example of a commercial system which permits heating of the sample is the JSTM-4200 microscope available from JEOL Corporation of Tokyo, Japan. Referring to FIG. 1, the sample 10, is mounted on a stage 12 which may be heated (or cooled by the passage of cryogenic fluids). The system is sealed and built from ultrahigh vacuum (UHV) compatible components. A scanning tip 14 and scanner 16 are all mounted inside (18) an ultrahigh vacuum (UHV) chamber 20. The absence of convective heating or cooling confines the heating or cooling to the sample. The small mass of the tip 14 limits heat transfer into the rest of the microscope via tip 14. Radiative heat transfer can be minimized by careful placement of baffles. While this system works well for its intended purpose, it is limited to use in a vacuum environment. Thus, many volatile samples and hydrated biological materials cannot be studied with this apparatus.
In order to solve this problem, Zhifeng Shao and co-workers have developed a microscope which operates in a cold cryogenic gas. It is described in U.S. Pat. No. 5,410,910 issued May 2, 1992 to A. P. Somlyo, Zhifeng Shao, Jian Xun Mou and Jie Yang. The layout of this microscope is shown schematically in FIG. 2. The microscope 22, consisting of a sample stage 24, tip 26 and scanner 28 is immersed inside a cryogenic Dewar 30. Microscope 22 is suspended by springs 32 which serve as part of a vibration-isolation system. The microscope is held close to the surface 34 of a cryogenic fluid 36 such as liquid nitrogen (LN.sub.2). In this way, the entire microscope is cooled. Condensation onto the microscope and the sample is avoided by operating in an environment of dry nitrogen gas 38. The system is preferably pressurized to prevent boiling of the LN.sub.2 and to ensure quiet operation of the microscope 22. This system has produced spectacular images of biological molecules, but it is cumbersome to operate. This is because the whole microscope must be kept at low temperatures. The sample and replacement scanning probe tips have to be passed into the Dewar via a transfer system (not shown) that prevents moist air from entering the Dewar. Accordingly, it is desirable to provide an improved STM/AFM device in which the sample may be heated or cooled, as desired, while providing easy access to tips, substrates and samples in a stable fashion so as to render atomic resolution images of heated/cooled specimens possible.